Pore pressure apparatus and method

ABSTRACT

This invention is for a method and an apparatus for determining the pore pressure of liquid in the ground and earth structures. This invention makes it possible to determine the pore pressure at various depths in the ground and earth structure. The apparatus can be positioned at the desired depth in the ground or earth structure and the pore pressure determined. The apparatus comprises means for increasing the pressure so as to counteract the pore pressure in the ground. Also, there is a control unit for measuring the pressure. More particularly, there is a diaphragm and a valve means. The valve means connects with the diaphragm. The pore pressure against the diaphragm is counteracted by the pressure from the control means. When the valve means is closed, the pressure is indicated and does not change. It is possible then to know the pore pressure in the ground.

United States Patent [72] Inventor PeturThordarson Kefavkik, Iceland[21] Appl.No. 648,824 [22] Filed June26,1967 [45] Patented Apr. 13, I971[73] Assignee LaucksLaboratories, Inc.

Redmond, Wash.

[54] PORE PRESSURE APPARATUS AND METHOD 5 Claims, 7 Drawing Figs.

[52] U.S.Cl 73/406 [51] Int.Cl................ G01l 7/08 [50]FieIdofSeai-ch.,.. 73/388, 406, 407, 94, 152, 420, 395, 396, 152,(Inquired) [56] References Cited UNITED STATES PATENTS 1,938,492 12/1933Moller 73/388 2,284,707 6/1942 Wilson... 73/388 3,134,260 5/1964Johnston 73/396 3,247,712 4/1966 Johnsonetal. 73/152 a /2 I f /8 g '14a} 35"." a; x 22 t e4- Primary ExaminerDonald O. WoodielAttorney-Christensen, Sanbom & Matthews ABSTRACT: This invention is fora method and an apparatus for determining the pore pressure of liquid inthe ground and earth structures. This invention makes it possible todetermine the pore pressure at various depths in the groundand earthstructure. The apparatus can be positioned at the desired depth in theground or earth structure and the pore pressure determined. Theapparatus comprises means for increasing the pressure so as tocounteract the pore pressure in the ground. Also, there is a controlunit for measuring the pressure, More particularly, there is a diaphragmand a valve means. The valve means connects with the diaphragm. The porepressure against the diaphragm is counteracted by the pressure from thecontrol means. When the valve means is closed, the pressure is indicatedand does not change. It is possible then to know the pore pressure inthe ground.

PORE PRESSURE APPARATUS AND METHOD This invention has many uses, as itcan be used to observe the progress of consolidation of embankments andfoundation soils which have been constructed by man. Also, thisinvention can be used-to determine the effective stresses for estimatingthe sheering strengths of embankments and slopes. Further, thisinvention can be effectively used to evaluate seepage through naturalground or embankments constructed by man. Further, the invention can beused to check the effectiveness of subsurface drainage facilities suchas natural subsurface drainage facilities like sand and gravel strata orartificially constructed subsurface drainage facilities such as pipes.Further, in the construction of earth dams, this invention can beprofitably used to determine stability control of the earth dam bothduring the construction of the earth dam and after the construction ofthe earth dam. Further, the invention can be used to determine theefficiency of the filter zones in earth construction and the use ofearth in construction. Also, the invention can be used to provide anindication of the embankment control in earth construction and, also, innaturally occurring earth formations. These above enumerated usesindicate, to a degree, some of the value of the subject invention.

This invention is related to the subject matter of the inventor'scopending Pat. application Ser. No. 643,584, filed Jun. 5, 1967, now US.Pat. No. 3,456,509 issued Jul. 22, 1969.

An object of this invention is to provide a pore pressure apparatuswhich will not corrode when placed and buried in the ground and in anearthen structure and subject to the liquids and chemicals in saidground and earthen structure; a further object is to provide a porepressure apparatus and method having a slight volume displacement so asto give a highly accurate reading of the pore pressure; another objectis to provide a pore pressure apparatus and method which is low in costand installation; a still further object is to provide a pore pressureapparatus and method based on a gaseous system wherein there is noliquid to entrap gas bubbles; another object is to provide a porepressure apparatus and method wherein it is not required to degas orremove gas from the system; an additional object is to provide a porepressure apparatus and method having equalizing pressure means runningdirectly from the liquid in the ground or earthen structure to theregion outside of the means for reading the pore pressure; an additionalobject is to provide an apparatus which is easy to operate and can bereadily operated; a further object is to provide an apparatus which hasnegligible time lag in the reading of the pore pressure; a furtherobject is to provide an apparatus which vmay be used in the verticalposition so that there is no water displacement; a still additionalobject is to provide an apparatus which when placed other than verticaldisplaces a very small quantity of water such as one-hundredth (0.0lcc.) of a centimeter of either water or liquid; and, a still furtherobject is to provide a pore pressure apparatus which can be used with aportable control case for determining the pore pressure.

These and other important objects and advantages of the invention willbe more particularly brought forth upon reference to the specificdisclosure of the invention, the appended claims and the drawings forthe apparatus.

In the drawings:

FIG. I is aschematic illustration of a pore pressure detector in theground or earthen structure and with the lines leading to the controlunit for indicating the pore pressure;

FIG. 2 is a longitudinal cross-sectional illustration of one species ofthe pore pressure detector and which shows the check valve in an openposition;

FIG. 2A, 'on an enlarged scale, is a fragmentary view of the check valveportion of the pore pressure detector and shows the check valve in aclosed position;

FIG. 3 is a longitudinal cross-sectional view of a second species of thepore pressure detector and illustrates the check valve in an openposition;

FIG. 3A is a fragmentary longitudinal cross-sectional view of the porepressure detector and illustrates the check valve in a closed position;

FIG. 4 is a longitudinal cross-sectional view illustrating a thirdspecies of the pore pressure detector and illustrates the check valve inan open position; and

FIG. 4A is a fragmentary longitudinal cross-sectional view of the porepressure detector and illustrates the check valve in a closed position.

In FIG. 1 there is illustrated a pore pressure detector 10. It is seenthat this detector 10 comprises a main body 12. The main body 12 has amain cavity I4.

The cavity 14 can be divided into four interconnecting cavities ofvarying cross-sectional area. For example, the uppermost cavity 16connects with the next lower cavity 18. It is seen that the lower cavity18 is of a smaller lateral crosssectional area than the cavity 16. Then,the cavity 18 connects with a cavity 20. The cavity 20 is of a largercross-sectional area than the cavity 18. And, the cavity 20 connectswith a cavity 22. The cavity 22 is larger in cross-sectional area thanthe cavity 20.

The cavity 22 is internally tapped at 24. A plug 26, externallythreaded, is screwed into the cavity 22. In this plug there are a numberof longitudinal drilled passageways 28. On the outer end of the plug andflush with the lower end of the body I2, there is a porous filter 30.Around the upper part of the plug 26, there are a number of radialpassageways 32.

It is seen that the upper or inner'end of the plug 26 bears against adiaphragm 34. The diaphragm is locked in place so that the main flexibleportion is in the cavity 20. In the sidewall of the body 12 there is adrilled passageway 36 and which passageway leads to and connects withone of the radial passageways 32. In assembling the pressure detector 10the plug 26 may be screwed into the tapped cavity 22 so that one of itsradial passageways 32 registers with and connects with the drilledpassageway 36. Also, there may be used a resin or an adhesive to firmlyand definitely position the plug 26 in the cavity 22. The drilledpassageway 36 may be considered to be a ground equalizing pressurepassageway.

A flexible plastic tube 38 connects with the ground equalizing pressurepassageway 36.

On the upper surface of the diaphragm 34, i.e., that surface away fromthe plug 26, there is attached a plunger 40. Said plunger 40 has a bodyportion 42 which rises into an upper larger part having a shoulder 44.The lateral cross-sectional area of the shoulder 44 is greater than thelateral crosssectional area of the cavity I8 and therefore the plunger40 will not move into the cavity 18. On the upper part of the plunger 40there is a shaft 46.

The plunger 40 may be attached to the diaphragm 34 by means of anadhesive or a resin.

In the lower part of the cavity 16 there is a tubular member 50. Thelower part of the tubular member 50 projects inwardly at 52 to form apassageway. Between the lower part of the tubular member 50 and thelower part of the cavity 16 there is positioned a quad ring 54. Theinner part of the quad ring 54 is of slightly less diameter than theinner part of the member 52. A sphere or ball 56 rests upon the innerpart of the quad ring 54. As is readily appreciated the quad ring 54 andthe sphere 56 form a check valve.

In the upper part of the cavity 16 there is a tubular member 60 which inturn defines a cavity 62. The cavity 62 registers at the cavity 16. Inthe upper part of the tubular member 60 there is a passageway 64.

The tubular member 60 is positioned in the upper part of the cavity 16and the lower end of the tubular member 60 bears against the upper partof the member 50. The tubular member 60 may be positioned in the body 12by means of an adhesive or a resin.

In the passageway 64 there is positioned a flexible plastic tube 66which may be identified as an incoming pressure tube.

It is seen that around the lower end of the tube 66 there is positioneda spring 68. The spring 68 bears against the ball or sphere 56 so as toclose the check valve.

The tube 66 may be sealed inside of the tubular member 60 by means of anadhesive 71. There is a spacer 73 around the lower end of the tube 66.The upper part of the spring 68 bears against the lower, surface of thespacer 73.

In the lower part of the body 12 there is a drilled passageway 70. Thispassageway connects with the cavity 20 and above the diaphragm 34. Ineffect, the passageway connects with the cavity 20, the cavity 18 andthe cavity 16.

A flexible plastic tube 72 connects with the drilled passageway 70. Theflexible plastic tube may be considered to be the outgoing pressuretube. The tube 72 may be positioned in the body 12 by means of anadhesive or resin 74.

In FIG. 2 it is seen that in the plunger 40 that there is a passageway76. This passageway 76 makes it possible for communication, when theshoulder 44 of the plunger 40 bears against the lower surface of thebody 12 around the passageway or cavity 18, between the cavity 20 andthe cavity 18.

The body 12, tubes 38, 66 and 72 are encased in a protective material 80such as a resin. The protective material 80 may be encased in a tubularplastic tube.

ln operation, and when the pore pressure detector 10 is buried in theground or the earth, the liquid flows through the porous filter 30, thepassageways 28 in the plug 26, and against the diaphragm 34. This movesthe diaphragm more into the cavity 20 and also moves the plunger 40 andthe shaft 46 toward the sphere 56. The sphere 56 is moved out of contactwith the quad ring 54 so as to open the check valve. At this time thereis sufficient pore pressure or liquid pressure to measure. A gas, suchas compressed air, is forced through the incoming pressure tube 66 andthis gas bears against the diaphragm 34. When the incoming gas in theincoming pressure tube 66 is sufiicient to move the diaphragm 34 awayfrom the sphere 56 the spring 68 bears against the sphere 56 so as toseat the sphere 56 on the quad ring 54 and to close the check valve.This prevents the flow of gas through the check valve, into the cavitiesl8 and 20, through the passageways 70 and through the outgoing pressuretube 72. At this instant the pressure in the tube 66 and the pressure inthe tube 72 will be the same. With an additional buildup of the pressurein the tube 66 there will be an additional buildup of pressure in thecavity 16 and the cavity 20. With the gauge for reading the pressure inthe tube 66 and a gauge for reading the pressure in the tube 72 it ispossible to detennine when the pressure in the tube 66 exceeds thepressure in the tube 72. When this occurs, a party then knows that thepressure in the tube 72 is indicative of the pore pressure or the liquidpressure of the soil. In this manner the pore pressure or the liquidpressure in the soil is determined.

In FIGS. 3 and 3A there is illustrated a pore pressure detector 100which is similar in construction to the pore pressure detector 10. Whereapplicable like reference numerals will be used for like components. Themain difference between the two pore pressure detectors l and 100 is inthe check valve mechanism.

In the lower end of the body 12 there is positioned a plug 102 which isexternally threaded. In this plug 102 there are longitudinal passageways104. The central portion of the plug 102, with respect to its inner end,is recessed to form a cavity 106. On the outer end of the plug 102 andin the body 12 there is positioned a filter 108. The filter 108 may be afine wire or a porous stone or other suitable material such as sinteredglass. There is a diaphragm l which has a central passageway 112. Thereprojects through the diaphragm 110 and in the central passageway 112 ashaft 114 which is threaded on its external end. The shaft 114 ispositioned on the diaphragm 110 by means of an upper nut 116 and a lowernut 118. The nuts 116 and 118 are on opposite sides of the diaphragm oron opposite surfaces of the diaphragm 110. The shaft 114 projectsupwardly into a head portion 120. On the head portion 120 there is agroove 122. An O-ring 124 is positioned in the groove 122.

In FIG. 3 it is seen that the diaphragm 110 has been moved upwardly andthat the O-ring 124 is out of contact with the curved throat portion 126of the cavity 62.

In FIG. 3A it is seen that the diaphragm is in a downward position andthat the O-ring 124 is now in contact with the curved throat portion 126to close the check valve.

in operation, and with liquid having passed through the filter 108 andthe passageways 104 and into the cavity 106 so as to bear against thediaphragm 1 10 thereby moving the shaft 114 upwardly and the O-ring 124out of contact with the walls and the throat 126, it is now possible tomeasure the pore pressure. A gas, under pressure, is allowed to enterthe tube 66. This gas bears against the upper surface of the diaphragm110 so as to urge the diaphragm 110 downwardly and to urge the shaft 114and the O-ring 124 downwardly. With sufficient pressure on the uppersurface of the diaphragm 110 the pore pressure is counterbalanced sothat the O-ring 124 bears against the curved throat 126 to close thecheck valve. Again with an increase in pressure in the tube 66 and thecavity 16 the pressure in the tube 66 exceeds the pressure in the tubes72. At this time it is possible, by reading the pressure in the tube 72,to know the pore pressure.

In FIGS. 4 and 4A there is illustrated a pore pressure detector 150. Thepore pressure detector comprises a lower body 152, an upper body 154,and an inner body 156. The lower body 152 comprises a lower cavity 158,a cavity 160, a cavity 162 above the cavity 160, and a tapped cavity 164above the cavity 162. In the wall of the lower body 152 there is apassageway 166 which connects on its inner end with the cavity 160. Itis seen that the lateral cross-sectional area of the cavity 158 isgreater than the lateral cross-sectional area of the cavity 160 so as toform a shoulder at the junction of the two. Also, the lateralcross-sectional area of the cavity 162 is greater than the lateralcross-sectional area of the cavity 160 so as to form a shoulder at thejunction of the two. Again, the lateral cross-sectional area of thecavity 164 is greater than the lateral cross-sectional area of thecavity 162 so as to form a shoulder at the junction of the two.

in the cavity 162 there is an inner body 156. The lateralcross-sectional area of the inner body 156 is slightly less than thelateral cross-sectional area of the cavity 162 so that the inner body156 will slide into and fit into the cavity 162. The inner body 156 hasa lower cavity and a cavity 172 positioned above the cavity 170 andconnecting with the cavity 170. There is positioned above the cavity 172a cavity 174 and which connects with the cavity 172. There is positionedabove the cavity 174 a cavity 176 which is positioned above the cavity174 and connects with the cavity 174. The diameter of the cavity 170 isgreater than the diameter of the cavity 172 and at the junction of thetwo there is a shoulder. The diameter of the cavity 172 is greater thanthe diameter of the cavity 174 and at the junction of the two there is ashoulder. The diameter of the cavity 176 is greater than the diameter ofthe cavity 174 and at the junction of the two there is a shoulder. Atthe bottom of the inner body 156 there is a circular recess 178 and atthe top of the inner body 156 there is a circular recess 180. Therecesses 178 and 180 define a lip.

On the bottom of the inner body 156 and positioned in the recess 178 isa diaphragm 182. This diaphragm is a downwardly directed diaphragm. Onthe upper part of the inner body 156 and positioned in the recess 180there is a diaphragm 184. This diaphragm is an upwardly directeddiaphragm.

In the cavity 172 there is a body member 186 which has a passageway 188.Below the passageway 188 there is a cavity 190 and above the passageway188 there is a cavity 192. In the cavity 190 there is an O-ring 194 andin the cavity 192 there is an O-ring 196. in the passageway 188 there ispositioned a sphere or ball 198. ln the cavity 172 there is positioned amember 200 having a longitudinal passageway 202 which connects with thecavity 190. In FlGS. 4 and 4A it is seen that the member 200 positionsthe body 186 in the cavity 172. Actually, the body 186 may bepositioned'in the cavity 172 by means of a resin or an adhesive and themember 200 may be positioned in the cavity 170 by means of a resin or anadhesive.

There is positioned in the cavity 174 and in the cavity 176 a shaft 204.The lower end of this shaft rests on the sphere 198.

The upper end of the shaft is free but is positioned close to a nut 206.The shaft 204 is in a spring 208. The lower end of the spring 208 restson the shoulder at the junction of the cavities 174 and 176. The upperend of the spring 208 bears against the lower surface of the nut 206.

In the diaphragm 184, in the center of the diaphragm, there is apassageway 210. A threaded shaft 212 projects through this passagewayand is screwed into the tapped opening of the nut 206. On the threadedshaft 212 there is an attached washer 214.

It is seen that the inner body 156 and that part enclosed by thediaphragms 182 and 184 and the passageways are filled with a liquid suchas a light oil.

The lower end of the upper body 154 has a cavity 220. There ispositioned above the cavity 220 a cavity 222. The cavity 222 connectswith the cavity 220. The upper part of the upper body 154 has a cavity224. The cavity 224 connects with the cavity 222. The lateralcross-sectional area of the cavity 220 is greater than the lateralcross-sectional area of the cavity 222 and at the junction of these twocavities there is a shoulder. The lateral cross-sectional area of thecavity 224 is greater than the lateral cross-sectional area of thecavity 222 and at the junction of these two cavities there is ashoulder.

In the lower end of the cavity 224 there is positioned a member 226 andwhich member has a passageway 228. Also, the member 226 rests on acircular ring 230. A quad ring 234 rests on the shoulder at the junctionof the cavities 222 and 224 and is inside of the circular ring 230 andis below the passageway 228. A sphere 236 is positioned in thepassageway 228 and also in the member 226. The sphere 236 is urged in adownwardly direction by means of a spring 238. The upper end of thespring 238 is positioned around a tube 240. The tube 240 is positionedin an upper member 242. The upper member 242 has a cavity 244 and whichcavity houses the spring 238 and the lower end of the tube 240.

The upper member 242 may be positioned in the cavity 244 by means of aresin or an adhesive. It is seen that the shaft 212 projects through thepassageway or cavity 222 so as to contact the sphere 236. Also, it isseen that the sphere 236 and the quad ring 234 comprise a check valve.

On the lower end of the pore pressure detector 150 there is a filter250. The filter 250 may be sintered glass or stone or other suitablematerial.

There connects with an upper part of the cavity 220 a cavity 252. Thereleads into the cavity 252 a tube 254. The inner end of the tube 254 ispositioned by means of a resin or adhesive 256. Then, the lower end ofthe tube 240 is positioned in the cavity 244 by means of a resin or anadhesive 258.

The tube 240 corresponds to the tube 66 of the pore pressure detectorand the tube 254 corresponds to the tube 72 of the pore pressuredetector 10.

In operation and with the pore pressure detector 150 buried in theground or in the earthen structure, the liquid such as water filtersthrough the filter 250, and some liquid enters through the passageway166. With the buildup in pressure in the cavity 160 the diaphragm 182 isforced upwardly or toward the sphere 198. The liquid 260 inside of thediaphragm 182 is forced and against the sphere 198 so as to urge thesphere 198 toward the shaft 204. The shaft 204 moves against the nut 206so as to urge upwardly the shaft 212. The shaft 212 unseats or removesthe sphere 236 from the quad ring 234 or from the check valve. Thisopens a means of communication between the tube 240 and the tube 254. Todetermine the pore pressure, the pressure in the tube 240 is increased.The reading of the pressure in the tube 240 and the reading of thepressure in the tube 254 are taken. With the continual increase in thepressure in the tube 240, and in the cavity in the member 226, the porepressure is finally equalized and exceeded so that the check valvecomprising the quad ring 234 and the sphere 238 is closed. The pressurein the tube 254 is then constant. With further increase in the pressurein the tube 240 there is a difference in the pressures in the tubes 240and 254. The pressure in the tube 254 is the pore pressure and can beread by means of a suitable gauge or other equipment.

The upper body 154 and part of the tubes 240 and 254 can be encased in aresin 262. This resin is a protective coat for the body and tubes. Also,there may be a tubular member 264 which encases the resin 262, the upperbody 154 and the tubes 240 and 254.

In FIG. 4 it is seen that the check valve is open andin FIG. 4A it isseen that the check valve is closed. I

In FIG. 1 there is schematically illustrated the control unit 270 forreading the pore pressure. The control unit 270 comprises a storage tank272 for storing air under pressure. There connects with the storage tank272 a pipe 274. A gauge 276 connects with the pipe 274. Also, the pipe274 connects with a valve 278. The valve 278 connects with a pipe 280. Apipe 282 connects with the pipe 280. Also, the pipe 282 connects withthe quick disconnect unit 284. The pipe 280 connects with a needle valve286. The needle valve 286 connects with a pipe 288. The pipe 288connects with a quick disconnect unit 290. A gauge 292 connects with thepipe 288. A gauge 294 connects with the quick disconnect unit 296. Apipe 298 connects with the pipe 288. The pipe 288 connects with a valveconnection 300. A safety valve 302 connects with the pipe 298.

The reserve tank 272 may be charged with air, under pressure, by meansof the quick disconnect unit 284. The quick disconnect unit 284 may beconnected to a source of air under-pressure so as to charge the tank272. Also, the tank 272 may be charged by means of a hand pumpconnecting with the valve connection 300.

In operation a hole 310 is drilled in the ground or in the earthenstructure. Then, the hole is partially filled with water. Then,approximately 12 inches of clean, medium to coarse sand 312 is placed inthe bottom of the hole. The pore pressure detector unit 10, or is placedon top of the sand. Then, approximately 24 inches of clean, medium tocoarse sand 314 is placed over the top of the pore pressure detector.Then, approximately 12 inches of bentonite balls 316 are placed in thehole to form a seal. The pore pressure detector is checked to determineif it works properly. The tube 66 or 240 is connected to the quickdisconnect unit 290. The tube 72 or the tube 254 is connected to thequick disconnect unit 296. The valve 278 is opened and also the valve286 is opened slightly. The pressure in the gauges 292 and 294 isallowed to increase approximately at a rate not to exceed 1 p.s.i. persecond. As previouslv stated, when the pressure of the gauge 292 exceedsthe pressure of the gauge 294 then the pore pressure is known. The porepressure is indicated by the pressure of the gauge 294. Then the holemay be filled with earth 318.

The materials of construction of the pore pressure detector 10, 100 and150 are of plastic. One of the plastics which can readily be used ismethyl methacrylate. The tube 38 may be plastic and approximatelyone-sixteenth inch in diameter. The tubes 66, 240, 72 and 254 are ofplastic and approximately one-eighth inchin diameter. These plastictubes can stand approximately 2000 p.s.i.

The pore pressure detectors 10, 100 and 150 have approximately a 2' inchoutside diameter. Also, these pore pressure detectors are approximately5 inches in length.

In the pore pressure .detector 100 the .volume of liquid displaced uponthe opening and closing of the check valve amounts to approximately 0.05milliliters. And, in the pore pressure detector 150 the volume of liquiddisplaced in the opening and closing of thecheck valve and also in theopening and closing of the sphere or ball 198 with respect to the O-ring196 and 194 is approximately 0.01 milliliters. In effect, it can be seenthat there is practically no displacement of the liquid to achieve areading of pore pressure.

With the use of a pore pressure detector made substantially of plasticand noncorrosive material, it is seen that there is substantially nochance of the detector becoming clogged and not giving a satisfactoryreading. With electrical connections, in a pore pressure detector, thereis considerable possiblity of the connections becoming corroded andfouled and not giving accurate readings or not giving readings. The tube38 in the pore pressure detectors l and 100 allow the displacement ofthe liquid to be accomplished without substantially no hindrance due tomovement of the liquid into the surrounding ground and earthen structureas there is freedom to move against atmospheric pressure. The passageway166 in the pore pressure detector 150 allows liquid to move from theinterior of the pore pressure detector to the exterior of the porepressure detector with the movement of the diaphragm 182 so as to makeit possible to realizean accurate reading.

lclaim:

l. A pore pressure detector comprising:

a body adapted to be placed under the ground surface having a firstcavity and a second cavity positioned therein, said first cavity beingin fluid communication with the liquid surrounding said body;

a diaphragm means separating said first cavity and said second cavity;

a fluid pressure means to impose an increasing pressure within saidsecond cavity;

valve means in said fluid pressure means to interrupt the pressurizationof said second cavity, said valve means operatively connected to saiddiaphragm so that when the force against said diaphragm from said secondcavity equals the force against said diaphragm from said first cavitysaid valve closes;

means to measure the pressure in said second cavity comprising apressure sensor in fluid communicating with said first chamber; and

fluid passageway means communicating with said first chamber to removegases from said first chamber.

2. The apparatus of claim 1 wherein said fluid passageway means isvented to the atmosphere.

3. The apparatus of claim 1 wherein said valve means comprises a sphereconstructed and arranged to engage a valve seat when closed.

4. The apparatus of claim 1 wherein said valve means comprises a movableO-ring and seat therefor.

5. The apparatus of claim 1 wherein said valve means is urged toward aclosed position by a spring means.

PU-1050 UNITED STATES PATENT OFFICE CERTEFICATE OF CORRECTION Patent No.3574284 Dated April 13, 1971 lnve fl Petur Thordarson It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Claim 1, line 18, should read --said second chamber, and-- Signed andsealed this 1 0th day of August 1971 SEAL) Attest:

EDWARD I I.FIETCHER,JR. WILLIAM E. SCHUYLER, J Attesting OfficerCommissionerof Patent

1. A pore pressure detector comprising: a body adapted to be placedunder the ground surface having a first cavity and a second cavitypositioned therein, said first cavity being in fluid communication withthe liquid surrounding said body; a diaphragm means separating saidfirst cavity and said second cavity; a fluid pressure means to impose anincreasing pressure within said second cavity; valve means in said fluidpressure means to interrupt the pressurization of said second cavity,said valve means operatively connected to said diaphragm so that whenthe force against said diaphragm from said second cavity equals theforce against said diaphragm from said first cavity said valve closes;means to measure the pressure in said second cavity comprising apressure sensor in fluid communicating with said first chamber; andfluid passageway means communicating with said first chamber to removegases from said first chamber.
 2. The apparatus of claim 1 wherein saidfluid passageway means is vented to the atmosphere.
 3. The apparatus ofclaim 1 wherein said valve means comprises a sphere constructed andarranged to engage a valve seat when closed.
 4. The apparatus of claim 1wherein said valve means comprises a movable O-ring and seat therefor.5. The apparatus of claim 1 wherein said valve means is urged toward aclosed position by a spring means.